I wanted to know the correct procedure for main earthing an electronics project built in a metal case, I live in the UK
Basically what Uncle Jed said.
An alternative would be to use an IEC socket and then the mains cable can be removed from the equipment. That is sometimes a useful feature.
Also the IEC socket could incorporate a filter to keep noise from entering the cabinet.
Any detachable panels on the cabinet/enclosure should be bonded to the earth connection with wire of the same cross section as the mains cable.
could someone please explain the difference between the different kinds of ground, that is - chassis ground, floating ground, mains ground etc
A bit of a minefield here with lots of confusing terminology.
I shall use some words which are familiar to me.
Earth - UK terminology
Ground - USA terminology
Both mean the same thing.
Safety Earth.
The standard yellow/green (UK) wire in the mains cable.
It is there for your safety, hence the name.
It should be connected to the chassis of the mains powered equipment, and so the documentation for the equipment may refer to mains earth or chassis earth.
Floating ground, 0Volt ground.
Sometimes in an equipment, the common connection of the power supplies, the 0 volt lines are not connected to the chassis.
This would be a floating 0v earth. All the internal signal would be connected to this 0v line, it would be their local common "ground", but it would not be connected to "Earth" ie the planet.
Some years ago, I worked on an offshore oil production platform.
The system which I was responsible for had four earth bars in the bottom of each cabinet, and they all had to be isolated from each other.
Safety Earth - connects all the cabinet metalwork together.
0v Earth - the reference connection for all the 5v, 12v and 24v DC power supplies in the cabinet.
Instrument Earth - an earth connection for all the screen of the cables going out from the cabinets to the instruments on the process.
I.S. Earth - a earth connection for all the Intrinsic Safety barriers. (Fitted to prevent hazardous voltages feeding out from the cabinet to areas where there may be explosive atmospheres)
Each of these four earth bars in each equipment cabinet were then connected by two cables to common earth bars for the module. ( The module was major chunk of the platform. there were something like seven modules to make up the platform).
Then the module earth bars were each connected by two cables to a common point on the Jacket. (The jacket is the steel structure which sits on the sea bed and holds everything up above the waves).
All in all a nightmare, which lead to some ridiculous steps having to be taken to ensure the isolation between earth bars was maintained.
So, there are more terms for earthing/grounding and weird and wonderfull schemes dreamed up by engineers with bees in their bonnets, than you can shake a stick at!
JimB